Photo-production of axions in Supernovae

Compact stellar objects like supernovae and neutron stars are believed to cool by emitting axions predominantly via axion bremsstrahlung ($NN \to NNa$), pion conversion ($\pi^- p^+ \to N a$) and photo-production ($\gamma N \to N a$). In this paper, we study in detail the photo-production channel, from the unavoidable anomaly induced Wess-Zumino-Witten term $\propto \epsilon^{\mu \nu \alpha \beta}\, F_{\mu \nu}\, \partial_\alpha a \, \omega_\beta$ in conjunction with the low energy pion photo-production data. We found that for heavier axions, i.e., $m_a\sim\mathcal{O}(100)$ MeV, photo-production processes can be dominant compared to the usual axion emission processes. In addition, the spectrum of axions emitted in the process is significantly harder than those originating from bremsstrahlung.